1. Field of the Disclosure
The present disclosure relates to electronic devices, more particularly, to electronic devices that include transistors having metal gate electrodes and a process for forming them.
2. Description of the Related Art
Many semiconductor manufacturing facilities run a mixture of products with different requirements in order to remain full and profitable. Thus managing cost and complexity trade-offs on a manufacturing line where a mixture of highly integrated products with different requirements are made can be particularly problematic and expensive.
For example, a substrate surface, as obtained from a commercially-available source, normally supports one transistor type. In order to form both a p-channel transistor and an n-channel transistor, another region of the substrate that includes a material that supports the other transistor type can be formed adding expense to the process. One method is to remove and replace portions of the substrate. If the substrate is a silicon-on-insulator substrate, another method is to expose a portion of the base layer and form the other transistor type there. Still another method is to deposit a SiGe layer that is greater than one micron thick over the substrate such that a surface of the SiGe layer opposite a surface of the SiGe layer closest to the substrate includes relaxed SiGe material. However, such methods require long processes be used that are both time consuming and expensive to perform. As an alternative to using long etch processes, the n-channel transistor and the p-channel transistors are formed at substantially different elevations. However, this adds complexity to both imaging and planarization processing during manufacturing.
As another example, threshold voltage (“Vth”) requirements can be different between high performance and low power components. High performance components have a low Vth to decrease the time for switching between states while low power components have a high Vth to reduce off-state leakage current. As such, high performance n-channel components have gate electrode work functions closer to the conduction band than the valence band of the channel region and high performance p-channel components have work functions closer to the valence band than the conduction band of the channel region. In low power components, a single gate electrode material with a work function approximately equally between the valence band and the conduction band can be used to reduce the processing cost over a two gate electrode process. Thus, if both high performance and low power components are integrated together on a single substrate, at least three separate gate electrode processes would have to be developed and maintained.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the invention. The use of the same reference symbols in different drawings indicates similar or identical items.